Make OOO_EXIT_POST_STARTUP behave more nicely when blank

Change-Id: I28bc8027908225d78c9543f3813d3bfe3818451d
Reviewed-on: https://gerrit.libreoffice.org/11264
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>